1. Field of the Invention
This invention relates to the general field of photonic band gap (PBG) fibers, more particularly to the excitation, propagation, control and use of various electromagnetic modes in PBG fibers and to structures of PBG fibers facilitating same. Most particularly, some embodiments relate to the acceleration and control of electrons moving axially along one or more PBG defects, leading to an improved source for electron beams having useful and improved characteristics, advantageous for a variety of applications.
2. Background and Related Art
The confinement and propagation of electromagnetic energy along fibers is a key technology in many important areas of the modern economy including communications, detection, sensing, probing (often remotely or within a patient for medical purposes), as well as many other areas of application. Perhaps the most common technique for confining electromagnetic waves within a fiber is total internal reflection, typically involving an optical fiber having a central axial strand of material surrounded by a cladding layer in which the central strand has a higher index of refraction (or “index”) than the index of the surrounding cladding layer. This arrangement of a high index axial strand surrounded by a low index cladding is constructed so as to cause electromagnetic waves propagating along the central strand and striking the strand-cladding interface at a glancing angle to undergo total internal reflection and thereby to remain propagating within the axial strand. The lower index of refraction in the cladding can be achieved by using a cladding material with inherently lower index than the material comprising the axial strand, or fabricating the cladding with numerous gaps, inclusions or other regions of low index such that the effective index of the total cladding structure is less than that of the axial strand.
However, the limitation that the cladding have a lower index of refraction than the axial strand in order to achieve confinement by total internal reflection is a serious limitation for many potential applications. For example, it would be advantageous to propagate a beam or cluster of electrons along a hollow central core (or “defect”) of a fiber-like structure concurrently with one or more confined electromagnetic modes such that the electrons gain energy from the electromagnetic mode(s). In such a structure, different modes can be used for bending, focusing and exerting other controls over the electrons. Unfortunately, the effective propagation of electrons requires a space free of material as electrons are scattered and/or captured by encounters with virtually any atom or molecule. No cladding material has a lower index of refraction than a vacuum, so a mode of confinement is required that allows electromagnetic mode confinement and propagation along a fiber having a defect region free of material.
Photonic band gap (PBG) fibers were developed in the 1990's to provide an alternative technique for confining electromagnetic waves within a defect region of an optical fiber. In essence, the defect region of an optical fiber (otherwise containing material with a relatively high index of refraction) can be hollow and air-filled, gas filled, evacuated, or partially evacuated, if it is surrounded by a structure having periodic variations in optical properties serving as the “cladding”. It is well known that when waves encounter a periodic structure, certain wavelengths will propagate through the structure while other wavelengths will not, analogous to the formation of electronic energy bands and band gaps that arise when electrons (having wave-like properties) interact with the periodic structure of a crystal lattice. That is, certain wavelengths (or ranges of wavelengths) will propagate through the periodic structure of the cladding and be lost to the propagation of the wave along the defect, while other wavelengths will lie in one of the (possibly several) wavelength “band gaps” and remain confined within the defect region of the fiber. Thus, electromagnetic waves having wavelengths in the range of a “photonic band gap (PBG)” will be confined to the defect region even though this core or defect region has an index of refraction lower than that of the surroundings. An extensive discussion and analysis of the propagation of electromagnetic modes through structures having periodic variations can be found in Photonic Crystals, 2nd Ed., J. D. Joannopoulos et al, (Princeton University Press, 2008), the contents of which is incorporated herein by reference for all purposes.
A typical PBG fiber is depicted in FIG. 1, taken from FIG. 4 of X. E. Lee, “Photonic Band Gap Fiber Accelerator,” Physical Review Special Topics—Accelerators and Beams, Vol. 4, pp. 051301-1, -7 (2001), hereinafter “Lin”. The entire contents of Lin is incorporated herein by reference for all purposes.
FIG. 1 depicts as 10 a dielectric material that includes an array of elements, 11, having different optical properties from the background 10 and are intended to create one or more band gaps, thereby preventing the propagation of electromagnetic modes having frequencies lying in the band gap(s). Elements 11 creating the band gap(s) are typically capillaries running axially through the fiber as depicted in cross-section in FIG. 1 and are referred to herein as “capillaries” or “band gap elements.” Material 10 is referred to herein as “background dielectric,” “dielectric material,” or simply “dielectric.”
However, since the central core is a distinct element of the fiber from those typically used as band gap elements 11, (such as a larger hole or absence of one or more band gap elements from an otherwise uniform fiber), analogous to a “lattice defect” as used in solid state physics, central core 12 is also referred to in literature as a “defect,” “core defect,” “central defect” and the like. These terms are typically used interchangeably to describe the central region of a PBG fiber, that is “central core,” “central region,” “defect,” “core defect,” and “central defect” are used without distinction. Essentially all fibers discussed herein are PBG fibers lacking high index material in the central core and will be so understood unless clearly indicated otherwise. Thus, it is customary in the field of PBG fiber technology to refer to the central core 12, having a different geometry from the surrounding capillaries 11, as the “central defect” or “defect.”
In addition, many of the PBG fibers considered herein pursuant to some embodiments of the present invention have more than one propagation region (defect), with some or all of such defects displaced from the central axis of the PBG fiber. Thus, “core” and “central core” and the like may carry the (erroneous) implication that the central axial region of the PBG fiber is intended when that is not necessarily the case. For clarity and economy of language we refer to such region(s) of propagation as the “defect” or “defects” understanding that a defect may, but need not, be located along the central axis of the fiber.
It is important to appreciate that, in contrast with the special modes discussed herein, PBG fibers used in telecommunications generally make use of electromagnetic modes largely confined to a PBG central defect, 12, for carrying information along the fiber. In contrast, the modes useful for different applications, such as electron acceleration, guidance and control as discussed herein, typically involve defect/surface modes in which the modes are not completely confined in the defect but in which important contributions to the performance of the PBG fiber arise from electric and magnetic fields (“fields”) lying outside the defect in the region of dielectric 10 and band gap elements 11. To be precise, we express the electromagnetic modes propagating axially along the PBG fiber (whether or not along the central axis) as propagating “in the region of, in the vicinity of, in the neighborhood of the defect,” reserving “in the defect” for those modes actually lying substantially within defect 12.
The creation, acceleration, control and use of electron beams by means of PBG fibers is one application for the technology described herein, and is expected to be an important practical example. In such cases, it is anticipated that laser light will be an advantageous source of the required electromagnetic energy. However, that is not an essential limitation and electromagnetic radiation outside the visible portion of the spectrum, and derived from sources other than lasers, are included within the scope of the present descriptions. For economy of language, “laser” or “light” is used herein to indicate general electromagnetic energy not necessarily limited to visible portions of the spectrum. Those with ordinary skills in the art will clearly realize when other wavelengths can be utilized for different purposes in appropriate circumstances.
The dielectric material 10 is depicted in FIG. 1 as a uniform background in which an array of other elements are embedded, typically band gap elements or capillaries, 11. While this is a typical structure for PBG fibers currently in use, it is not a fundamental limitation. Regions of different material having different optical properties can also be employed in place of a substantially uniform background dielectric 10, providing additional design parameters for making the properties of the PBG fiber precisely as desired. However, to be concrete in our descriptions, we describe the typical case in which 10 represents a substantially uniform dielectric material.
The periodic array of band gap elements or capillaries 11 is depicted as a hexagonal array in FIG. 1, but that is not an essential limitation. A hexagonal pattern provides advantageous packing or close packing for the arrangement of capillaries 11, and also is conveniently manufactured with present fiber fabrication technology. To be concrete, many of the descriptions herein depict or describe hexagonal patterns for capillaries 11, but other arrangements, such as square, may also be used advantageously in some cases, and are included within the scope of the present descriptions.
Central defect 12 as depicted in FIG. 1 denotes the central, axial region of the PBG fiber within which, or within the vicinity of which, electromagnetic radiation with appropriate wavelength(s) typically propagates (at least for those cases lacking multiple defects). To be precise in our terminology, we use “strand” or “central strand,” “axial strand” and the like to indicate the central light-carrying region of a conventional optical fiber confining light by means of internal reflection at the strand-cladding interface with a higher index strand surrounded by a lower index cladding. In other words, “strand” or phrases including “strand” are used herein to denote a light-carrying fiber structure having material with relatively high index of refraction along its central axis. We distinguish the central region of PBG fibers as “central defect (core),” “central defect (core) region,” and the like to indicate the central axial region of a PBG fiber lacking high index material, typically evacuated or partially evacuated, but may optionally contain low index material such as air or other gases.
The mechanism confining electromagnetic radiation to the vicinity of the central core of a PBG fiber does not require material to be present in the core, so one may envision including within the core substances that interact with the confined radiation to produce advantageous results. For example, Lin proposes that a properly constructed PBG fiber having radiation propagating along the central core has the potential to provide an effective electron accelerator. Whereas conventional electron accelerators are capable of adding energy to the accelerated electrons at about 50 MeV/m (50×106 electron volts per meter), even estimating performance of superconducting accelerators, a PBG fiber accelerator (“PBG accelerator”) has the potential to impart energy at the rate of more than about 1 GeV (109 ev)/m. Thus, PBG accelerators may provide a very compact, perhaps portable, accelerator.
To be concrete in our descriptions, we presume that electrons are the particles to be accelerated in a PBG accelerator, understanding thereby that this is by way of illustration not limitation since any charged particle in the PBG's defect region will interact with the electromagnetic fields therein, potentially producing useful effects. In particular, positive charged electrons (positrons) can make use of PBG accelerators in a manner very much like electrons and with the same structure as an equivalent PBG electron accelerator. Positrons are already useful in medicine, for example, in positron emission tomography.
Clearly, it is important to be able to insert electromagnetic energy into the defect region of a PBG fiber in sufficient quantity and having the desired electromagnetic field structure. In other words, electromagnetic energy must be coupled into the fiber in such a way so as to excite the electromagnetic modes desired and do so as efficiently as is reasonably possible.
Thus, a need exists in the art for improved structures, devices, materials and procedures for exciting, propagating and controlling various electromagnetic modes with defect region(s) of a PBG fiber so as to produce desired effects therein, including acceleration and control of charged particles, for an improved source of electron beams having one or more advantages of high energy, compactness, low cost, among others.